The herpes simplex virus 1 (HSV-1) genome encodes at least 76 genes (McGeoch, et al., 1985; Chou, et al., 1986; McGeoch, et at., 1988; Liu, et al, 1991; Barker, et at., 1992). Inasmuch as three of the genes are contained within repeated sequences, the genome encodes at least 73 polypeptides. The 76 genes comprise several groups designated as .alpha., .beta. and .gamma., whose expression is coordinately regulated and sequentially ordered in a cascade fashion (Honess, et al., 1974). For the past two decades, this laboratory has investigated the mechanisms by which HSV-1 regulates the expression and function of its genes. In the course of these studies, we demonstrated that (i) initiation of transcription of viral genes is induced by a structural component of the virion (Post, et at., 1981; Batterson, et at., 1983), (ii) regulatory proteins induced by this process are phosphorylated (Pereira, et at., 1977; Marsden, et al., 1978) and (iii) binding of these proteins to DNA is affected by phosphorylation (Wilcox, et at., 1980). Since protein kinases (PK) are known to affect the function of regulatory proteins, we sought to identify the substrates of viral PKs and clarify their function.
Both HSV-1 U.sub.S 3 and U.sub.L 13 gene products contain motifs common to known protein kinases (McGeoch, et at., 1985; McGeoch, et at., 1988; McGeoch, et at., 1986; Smith, et at., 1989). Definitive evidence of U.sub.S 3 PK activity emerged from the observations that (i) the novel enzyme activity detected in cells infected with wild type virus was absent from uninfected cells and cells infected with a mutant from which the U.sub.S 3 gene had been deleted (Purves, et al., 1986; Frame, et al., 1987), and (ii) antibody raised against a synthetic eight-amino acid C-terminal U.sub.S 3 oligopeptide reacted with purified preparations of the enzyme (Frame, et al., 1987). Recently, we demonstrated that the most prominent target of the U.sub.S 3 PK is an essential, non glycosylated membrane protein encoded by a highly conserved herpesvirus gene, U.sub.L 34 (Purves, et al., 1991; Purves, et al., 1992). In the absence of U.sub.S 3, the nonphosphorylated U.sub.L 34 protein associates with several phosphorylated proteins not demonstrable in cells infected with wild type virus. Concurrently, it was shown that the U.sub.S 11 protein may act as an antiterminator of transcription of U.sub.L 34 inasmuch as (i) significant amounts of a truncated U.sub.L 34 mRNA accumulate in cells infected with a U.sub.S 11.sup.- virus and (ii) the U.sub.S 11 protein binds to U.sub.L 34 mRNA immediately 5' to the truncation (Roller, et al., 1991). Although the functions of the U.sub.L 34 protein remain unknown, the regulation of its synthesis and expression deduced from the activities of U.sub.S 11 and U.sub.S 3 protein attest to its role in the reproductive cycle of the virus. Very little is known about the targets of the U.sub.L 13 PK. To investigate the role of U.sub.L 13 in the processing of the U.sub.L 34 protein, we constructed viral mutants lacking either the U.sub.L 13 gene or both the U.sub.L 13 and U.sub.S 3 genes. We report that a substrate of U.sub.L 13 PK is the product of the regulatory gene, .alpha.22.